1. Field of the invention
The present invention relates to a light emitting device and a manufacturing method of the same. More specifically, the present invention relates to a light emitting device having LED chips mounted on a mounting substrate sealed with a glass material, and a manufacturing method of the same.
2. Description of the related art
Conventionally the following light emitting device has been developed: LED chips are flip-chip bonded on a mounting substrate, and a plate-shaped glass material (glass plate) is hot pressed on the mounting substrate, so that the glass plate is taken as a glass sealing member and the LED chips are glass sealed (see: Patent References 1 through 3).
For this light emitting device, a plurality of LED chips can be sealed en bloc by the glass sealing member, so that the productivity is good.
It is also possible to avoid heat damage of the LED chips caused by overheat, as glass sealing can be carried out in a short time near the softening point of the glass plate.
According to the technology disclosed in Patent Reference 2, a powder mixture is prepared by mixing a powdery glass material and a powdery phosphor; after the powder mixture is melted, the powder mixture is solidified to form a phosphor-dispersed glass; the obtained phosphor-dispersed glass is used as the glass sealing member, so that the phosphor is pumped by the light emitted from the LED chip to generate a wavelength converted light; as a result, it is possible to reduce uneven color and, at the same time, to increase the light emitting efficiency.
According to the technology disclosed in Patent Reference 3, a glass prepared by melting a powdery glass material followed by solidification is used as the glass sealing member, so that fine voids are dispersed inside the glass sealing member; as a result, the light emitted from the LED chip is diffused by the voids; consequently, it is possible to increase the efficiency in output of the light.
Patent Reference 1:WO2004/082036
Patent Reference 2:JP-A-2008-071837
Patent Reference 3:JP-A-2008-270563
In the method for forming the glass sealing member, first of all, a glass plate is superposed on the upper surface (the mounting surface of the LED chips) of the mounting substrate where the LED chips are flip-chip bonded; then, while the mounting substrate and the glass plate are superposed with each other, the mounting substrate and the glass plate are set between the lower die and the upper die of a hot press processing device; then, in the sealing atmosphere, while the lower die and the upper die are heated, hot press operation is carried out by pressing the mounting substrate and the glass plate; as a result, the glass plate is hot-press bonded on the glass plate, and the LED chips are glass sealed on the mounting substrate by the hot softened glass plate; the LED chips are glass sealed in this way.
Here, the glass plate is prepared by solidifying a glass material in a fully melted state, and it has a smooth surface. In addition, the inner surface of the upper die is also smooth.
Consequently, when hot press processing is carried out, the sealing atmosphere enters between the upper die and the glass plate. The sealing atmosphere is left in between the upper die and the glass plate, because there is no way for the sealing atmosphere to escape. As the glass plate is hot pressed on the mounting substrate in this state, dips caused by lumps of the residual sealing atmosphere (known as “air trap”) are formed on the surface of the glass plate, degrading the flatness of the surface of the glass plate as the glass sealing member, which is undesirable.
If dips are formed on the surface of the glass sealing member and its flatness degrades, light emitted from the LED chips cannot be normally output, so that the light emitting efficiency of the light emitting device decreases. As a result, the light emitting device becomes a defective product, degrading the yield.